Zn-rich smectite from the Silver Coin Mine, Nevada, USA

Abstract

More than 100 minerals have been reported from the Silver Coin Mine, Nevada USA; five new minerals have been discovered here, due to the unusual geochemical environment. The present study reports on the investigation of a greenish clayey sample from the Silver Coin Mine. After the separation of a fine fraction to enrich the clay minerals, sauconite, a rare Zn-rich smectite, was found by X-ray diffraction (XRD) and was further characterized by differential thermal analysis (DTA), infrared (IR) spectroscopy and scanning electron microscopy (SEM). The Zn-rich smectite is accompanied by illite, minor kaolinite/halloysite and traces of gibbsite (as was indicated by the IR spectroscopy). The occurrence indicates an acidic environment probably caused by oxidation of sulfides.The determination of the structural formula, to further characterize the Zn-rich smectite, was difficult because of the multi-clay mineral assembly. However, different SEM-EDX (energy dispersive X-ray) approaches as well as transmission electron microscopy (TEM)-EDX analysis helped to characterize the smectite as Al-rich sauconite with some exchangeable K+.

New insights into the crystal chemistry of sauconite (Zn-smectite) from the Skorpion zinc deposit (Namibia) via a multi-methodological approach

“…The curves from TGA-DTA-DTG experiments are shown in Figure 5 and the relevant results compared with literature data are listed in weight loss. This two-step dehydration is in agreement with previous works (Kaufhold et al 2015) and consistent with the two-layer hydration of the sauconite studied here, testified by the (001) d- spacing from XRPD analysis (Moore and Reynolds 1997). The broad band around 93°C could be due to the loss of half of the water molecules linked to the interlayer cations, accounting for the change from bi-layer to mono-layer states, as defined by Ferrage et al (2007aFerrage et al ( , 2007b.…”

“…X-ray diffraction measurements on oriented mounts after air drying (AD), ethylene-glycolation (EG) and thermal treatments were carried out following the procedure in Reynolds (1980) and Moore and Reynolds (1997). The comparison of the AD and EG pattern in Figure 3 clearly evidences a shift of the 001 peak from ~14.9 Å in the AD pattern to 16.8 Å in the EG sauconite, as expected for this mineral (see also Higashi et al 2002;Yokoyama et al 2006;Coppola et al 2008;Kaufhold et al 2015;Choulet et al 2016). Moreover, the 001 peak is centered at 15.1 Å in the pattern collected after heating at 400°C, whereas it shifts to 9.6 Å when the sauconite is heated at 550°C as a consequence of the shrinking of the smectite along the stacking direction (Jackson 1979;Higashi et al 2002).…”

“…Finally, the exothermic peak at 820°C may be associated to the breakdown of sauconite and crystallization of new phases (see also Faust 1951;Higashi et al 2002;Petit et al 2008;Kaufhold et al 2015, and Table 2). There is no complete agreement in the literature about the phases formed upon heating.…”

“…Ross 1946;Faust 1951;Newman and Brown 1987;Kärner 2006;Coppola et al 2008;Kaufhold et al 2015;Mondillo et al 2015;Buatier et al 2016;Choulet et al 2016;Balassone et al 2017Balassone et al , 2020) and of synthetic sauconite (i.e. Decarreau et al 1987;Higashi et al 2002;Yokoyama et al 2006;Petit et al 2008;Pascua et al 2010;Zhou et al 2017), only few of them (Faust 1951;Coppola et al 2008;Kaufhold et al 2015;Choulet et al 2016) report some structural features of this Zn-smectite. This lack is essentially due to the intrinsic complexity of sauconite, caused by the occurrence of stacking disorder, common in layered minerals (Lanson 2011), as well as by the variable extent of cation substitutions, different coordination and positional disorder of interlayer cations, variable number of water molecules and different interlayer water configurations.…”

“…Even though several studies have been published on the chemical characterization of natural sauconite from different deposits (i.e. Ross 1946;Faust 1951;Newman and Brown 1987;Kärner 2006;Coppola et al 2008;Kaufhold et al 2015;Mondillo et al 2015;Buatier et al 2016;Choulet et al 2016;Balassone et al 2017Balassone et al , 2020) and of synthetic sauconite (i.e. Decarreau et al 1987;Higashi et al 2002;Yokoyama et al 2006;Petit et al 2008;Pascua et al 2010;Zhou et al 2017), only few of them (Faust 1951;Coppola et al 2008;Kaufhold et al 2015;Choulet et al 2016) report some structural features of this Zn-smectite.…”